With regard to the first use of this invention, various compositions have been used as protective coatings, containing polymers such as phenolic resins, polyesters, polyurethanes, epoxy resins and polyvinyl chloride resins, which also contain metallic chromates or phosphates, metallic oxides and/or zinc in particulate form. Zinc-rich primers have been considered to be optimum anti-corrosion coatings on iron or steel substrates. However, certain problems have restricted their use as industrial type primers. The action of zinc powder in inhibiting rust is based on an electrochemical interaction between the zinc and the steel substrate. In order not to insulate the zinc particles from each other and from the substrate the prior art has considered it necessary to use very little binder, with satisfactory corrosion protection achieved only when the zinc:binder ratio is at least about 9:1. The high zinc level and the relatively high density of zinc powder often cause undesirable settling during short term storage. Hence, the zinc powder is often added just prior to application and mixed rapidly during application to prevent settling and clogging of spray equipment. This deters efficient field use.
A low binder content was previously considered to be an advantage since a higher binder level would isolate the metallic grains from each other and from the substrate to be protected, thereby providing ineffective cathodic protection.
A lower zinc content is disclosed in U.S. Pat. No. 3,998,771, issued December, 1976 to T. J. Feneis, Jr. et al, which describes water-based coating compositions for application on iron supports to obtain anti-corrosive coatings. Single phase compositions in this patent include about 2% to 10% by weight of a non-volatile liquid epoxy resin, with low viscosity, derived from bisphenol A and an epihalohydrin, e.g., epichlorohydrin; about 2% to 10% by weight of a modified polyamide, i.e., an addition product of a water soluble polyamide and a liquid epoxy resin; and about 55% to 70% by weight of a zinc-powder pigment having an average particle size of about 2 to 15 microns.
U.S. Pat. No. 4,417,007, issued November 1983 to G. A. Salensky et al, discloses a one component composition containing from about 4% to 25% by weight epoxy or phenoxy resin binder and a polyamine hardener, about 43% to 90% by weight zinc dust, about 3% to 38% by weight Mn.sub.3 O.sub.4 fume pigment, up to 35% by weight additional pigments including pigment extenders and fillers (such as talc, clays, diatomaceous silica and silica), up to 5% by weight pigment suspension agent (such as hydrous magnesium silicate and lecithin), and balance organic solvents. A 1:1 volume ratio of zinc dust:Mn.sub.3 O.sub.4 is preferred.
U.S. Pat. No. 4,891,394, issued January 1990 to the applicant of the present invention, discloses a coating composition for the protection of metallic and non-metallic substrates against environmental attack, comprising about 10% to about 25% by weight of a film-forming polymer which may be epoxy resins having an epoxide value of about 250 to 2500, vinyl chloride resins copolymerized with polyisocyanates, and/or vinyl chloride resins copolymerized with melamines; about 30% to about 60% by weight particulate metallic zinc; an agent for control of electrical conductive characteristics comprising a crystalline silica having an oil absorption value of less than 20 as measured by ASTM Test D281-84, the volumetric ratio of such agent to the metallic zinc ranging from about 0.7:1 to about 1.25:1; about 2% to about 3% by weight of an agent for control of rheological characteristics comprising a pyrogenic amorphous silica having an average particle size less than about 0.012 micron; and at least one solvent compatible with the polymer.
French patent application 2,602,239, published Feb. 19, 1988 in the name of the applicant of the resent invention, discloses a two phase coating composition containing up to 70% by weight of a powdered metal (based on the total weight of the composition after admixture), from about 2% to 30% of a film-forming polymer, about 2% to about 30% of a hardener for the polymer, at least 1.8% to 30% of an agent for control of rheological characteristics, and up to 30% by weight organic solvents. The preferred polymer is an epoxy resin having an average molecular weight of 350 to 3800. The agent for control of rheological characteristics comprises at least one pyrogenic silica and optionally at least one natural or transformation silica having an oil absorption value preferably not greater than 90 and more preferably not greater than 40. In the specific examples, pyrogenic silicas were used having average particle sizes of about 0.014 micron, about 0.007 micron and about 0.008 micron.
U.S. Pat. No. 4,748,194, issued May 1987 to Geeck, discloses a coating composition for the protection of gas tanks, comprising a powder metal (such as zinc, cadmium, stainless steel, aluminum, alloys or mixtures thereof), a linear epoxy or phenoxy resin having a molecular weight of less than 15,000 cross-linked with a blocked isocyanate, a suspension agent, a thixotropic agent, and "active" and "inactive" organic solvents. The proportion of powdered metal present ranges from 13 to 52 parts per hundred. The suspension agent disclosed in this patent is polyethylene, and the thixotropic agent is silane treated silicon dioxide, in amounts up to 2 parts per hundred.
U.S. Pat. No. 4,621,024, issued Nov. 4, 1986 to F. A. Wright, discloses metal coated microspheres and a process for preparation thereof. Particulate zinc, aluminum, silver, copper, stainless steel, platinum, gold, or mixtures thereof, having an average particle size of about 6 to 10 microns, are bonded to the surfaces of non-conductive microspheres by means of a thermosetting adhesive coating on the microspheres with application of heat, followed by intermittent mixing in the absence of heat. The microspheres may be fly ash, comprising about 80-96% by weight alumina-silica, with minor amounts of iron oxide, alkaline earth metal oxides and alkali metal oxides. The adhesive binder preferably comprises an organo-functional silane and a copolymerizable monomer. In the final product the metal is from about 15% to about 30% by weight, relative to the weight of the adhesive binder-coated microspheres. Although this patent discloses average particle size diameters of metal coated microspheres ranging from about 60 to 180 microns, the assignee of this patent also produces zinc coated microspheres of smaller average diameters, e.g., about 2.5 to about 60 microns.
The use of zinc-coated microspheres disclosed in the above mentioned U.S. Pat. No. 4,621,024 in zinc-rich inorganic binder compositions has been proposed by the prior art, as a partial replacement for zinc dust. More specifically, substitution of between 20% and 40% by volume of zinc-coated microspheres, for the zinc dust, has been evaluated in a silicate primer (produced by Carboline Company of St. Louis, Mo., under the trademark "Carbo Zinc 11"). Silicate binders of this type have a very slow drying time, and also require blast cleaning of the metal substrate prior to deposition of the coating. These coatings are electrically conductive.
Co-pending allowed patent application U.S. Ser. No. 07/639,782, filed Jan. 10, 1992, by the applicant of the present invention, now U.S. Pat. No. 5,182,318 discloses a coating composition exhibiting improved resistance to corrosion of metallic substrates. The glass microspheres in the '782 application are coated with zinc and are concentrated primarily at the exposed surface of a dry coating, so as to provide corrosion protection to the substrate. The '782 application provides suitable conditions (by solvent selection) for the hollow glass microspheres to rise to the surface of the coating, to provide the desired protection to the substrate.
Glass microspheres, not coated with a metal, heretofore have been used primarily in non-paint related uses such as: polymeric panels which form a part of airplanes; syntactic foams, electrical potting compounds, randomes in the aerospace industry; syntactic foams in the hydrospace industry; plastisols, adhesives, polymeric spare parts in the automotive industry; wall repair compounds, caulks, sealants and tape joint compounds in the construction industry; in increasing the velocity of detonation, optimum sensitization and chemical stability of industrial explosives; as part of sporting goods such as tennis rackets, flyfishing lines, bowling balls and golf balls; as trowling mix and putty for the marine market; and other applications.
In all of the above listed uses, some of the characteristics of "non-coated" microspheres which are of greatest significance are: the lightness (weight) and resulting lower composite density; spherical shape; inherent strength because of the sphericity, as compared to other fillers; cost effectiveness due to the lower composite density (cost is even lower for the "non-coated" microspheres compared to the metal-coated microspheres); chemical resistance; excellent moisture resistance; low dielectric constant; low electric conductivity; decreased application and drying time, etc.
Co-pending patent application Ser. No. 07/972,115 filed on November 5, by the applicant of the present invention, discloses an improved low-cost coating composition for use in non-gloss and low-gloss applications which require high, dry deposition thicknesses, said composition comprising, apart from the film-forming polymer and the volatile components, from about 5 to 30 volume percent of lightweight, hollow, glass microspheres, having diameters ranging from 1 to 150 microns.
As demonstrated by the '782 application and the "architectural" application, many of the above properties exhibited by these microspheres could be beneficially used in coatings-related applications. However, manufacturers such as 3M and PQ corp., make it clear that their sales of the "non-coated", hollow, glass microspheres in the past 12 years to the coatings industry have been insignificant, at best. The principal reason for this lack of enthusiasm for the lightweight microspheres is that their low specific gravity (specific gravity generally ranges from 0.1 to 0.6), causes the spheres to float to the surface (caking), making commercial exploitation rather difficult, except where this particular property is specifically desired and appropriately exploited, as in the '782 application.
With respect to both uses of the present invention, it is important that the final coating itself be a good conductor (have a conductivity of at most 3 ohms/cm.sup.2). Conventional wisdom would therefore point away from using the "non-coated" microspheres, because of their low electrical conductivity.
The present invention, circumvents these possible problems, thereby making it possible for the coatings industry to harness the potentially tremendously advantageous properties displayed by these microspheres, listed above. This invention solves the "lightness" problem with the use of (1) appropriate wetting agents; (2) appropriate dispersants;(3) appropriate chemical thickeners; and (4) glass microspheres with specific gravity greater than water. It is believed that the low electrical conductivity problem is overcome by (a) the inherent tendency of the microspheres to occupy a much smaller volume within the coating, as compared to conventional extender pigments and (b) the naturally low oil absorption displayed by the microspheres. It is believed that these two characteristics of the "non-coated" hollow glass microspheres, allow the free movement of metal in a liquid phase between the spheres, thereby not restricting the overall conductivity of the coating. Conductivity remains virtually unchanged from relative volume levels of 1 volume metal:1 volume spheres to 1 volume metal:1.5 volume spheres.
In conventional cathodic and conductive coatings, typical extender pigments used are talcs (such as hydrated magnesium silicate) with an oil absorption of 45-60%; diatomaceous silica (such as celite) having greater than 50% oil absorption and fumed or pyrogenic silica, having oil absorption between 100 and 400%. The presence of these pigments causes an increased absorption of pigments onto the substrate surface. This results in the reduction of the critical pigment volume concentration. Pigment volume concentration (PVC) is the ratio of the pigment-volume to the total volume of the non-volatiles. It can be best expressed by the formula: ##EQU1## In a coating composition with low pigmentation, each pigment particle is assumed to be surrounded by binder. As the pigmentation increases, at a certain PVC, there is just enough binder to barely cover the pigment and fill the interstices. This PVC is called the critical PVC or CPVC.
Exceeding the CPVC results in voids and in the film becoming porous. Tensile strength, flexibility, corrosion resistance and water-vapor impermeability, all decrease. Thus if CPVC decreases, CPVC can be more easily exceeded, as with the above listed high oil absorption pigments. Hence, poor protection to the substrate results. The present invention solves these problems by utilizing light weight hollow glass microspheres, which have extremely low levels of oil absorption to partially or completely replace traditional pigments, thereby maintaining a high enough CPVC than would be possible, if one were to use pigments with high levels of oil absorption.
With regard to the second use of the invention, namely low-cost organic, conductive coatings for RFI and EMI shielding, it is well known that most electronic equipment including, computers, aerospace and military communications equipment (e.g., the "fly by wire" control systems common to modern aerospace computers) are subject to interference caused by numerous extraneous signals that can negatively affect the proper functioning of the electronic equipment. For example, such a shortfall in the anticipated performance of aerospace computers could result in the loss of aircraft and the failure of NASA projects.
Interference is even more pervasive when the cabinets which hold the electronic equipment, are made of the various plastics now in common use by the electronics industry for reducing weight and improving design capabilities. As these plastic housings are non-conductive, they do not provide a natural shielding to the computers from RFI and EMI. In order to eliminate or reduce this problem and to assure reliable functioning of important electronic equipment susceptible to RFI and EMI, effective shielding must be provided.
In recent years, surface coatings, either applied directly to the electronic equipment or applied to the plastic housings of the equipment, have been an important means of providing effective shielding from RFI and EMI. Organic coatings are preferred within the electronic industry, because of their cost effectiveness. Conventional organic coatings are non-conductive and would present the same problem as that faced by the plastic housings. Hence, conductive coatings were developed to provided proper shielding.
U.S. Pat. No. 4,522,890 ('890) issued Jun. 11, 1985 to Volkers et al., discloses a composite structure of thin films applied by vacuum deposition, including alternating layers of high conductivity metals and low conductivity metals to combine the effects of reflection and absorption and thereby maximize the reduction of the EMI/RFI signal strength as the signal passes through the thin films. Additionally, a similar structure of layers of materials with differing magnetic permeabilities may be used for the same purpose. The '890 patent itself points out some of the disadvantages of the technology disclosed therein. The shielding effect of thin films is acknowledged to be more effective on plastic or non-conductive substrates than on conductive substrates.
Additionally, the '890 patent reveals some of the problems associated with using traditional organic coatings which were designed to be conductive, by the use of a high conductivity metal powder such as copper or silver. While the EMI/RFI signal strength does reduce upon passing through such a coating, the coating itself is subject to adhesion problems and problems associated with the metal sinking to the substrate during drying (due to its high specific gravity) and thus causing a non-uniform coating. The settling during drying also raises the specter of the problems associated with zinc-rich coatings during storage. As described earlier, use of metal powders in conductive coatings results in settling of the metal during short-term storage and hence would deter efficient field use.
The problem of poor adhesion to non-metallic substrates, displayed by metal rich conductive coatings is overcome because of the extremely low oil absorption of the glass spheres. The problem of settling of the metal powder in both the cathodic and the conductive coating uses of the present invention is overcome, again due to the extremely low oil absorption of the microspheres as well as the low density of the microspheres.
Plastic materials which are conductive themselves have also been disclosed for use in manufacture of housings of electronic equipment susceptible to EMI/RFI. U.S. Pat. No. 4,765,928 issued Aug. 23, 1988 to M. Thakur, describes polymers having an intrinsic on conductivity of 10.sup.-9 to 10 mhos/cm, the polymers being "doped" with an electron donor or an electron acceptor dopant. Frequently, increasing the conductivity of these plastic materials compromises the strength and other characteristics such as design capability, which led to the use of plastics as the material for the housings.
As discussed above, the use of "non-coated", hollow, glass microspheres, instead of conventional pigments, in cathodic and conductive coatings, results in unaffected overall electrical conductivity; lower overall specific gravity of the coating; improved tensile strength, flexibility, corrosion resistance, water-vapor impermeability; and lower chances of "mud-cracking".
Having described the problems associated with prior art cathodic and conductive coatings, and the problems associated generally with the attempt to use the lightweight glass microspheres in coatings, the objects and aspects of the invention will be stated next.